High-affinity auxin transport by the AUX1 influx carrier protein

In plants, auxin is a key regulator of development and is unique among plant hormones in that its function requires polarized transport between neighboring cells to form concentration gradients across various plant tissues [1-5]. Although putative auxin-influx [6, 7] and -efflux [8-15] transporters have been identified by using molecular genetic approaches, a detailed functional understanding for many of these transporters remains undetermined. Here we present the functional characterization of the auxin-influx carrier AUX1. Upon expression of AUX1 in Xenopus oocytes, saturable, pH-dependent uptake of H-3-IAA was measured. Mutations in AUX1 that abrogate physiological responses to IAA in planta resulted in loss or reduction of H-3-IAA uptake in AUX1-expressing oocytes. AUX1-mediated uptake of H-3-IAA was reduced by the IAA analogs 2,4-D and 1-NOA, but not by other auxin analogs. The measured K-m, for AUX1-mediated uptake of H-3-IAA was at concentrations at which physiological responses are observed for exogenously added IAA and 2,4-D. This is the first report demonstrating detailed functional characteristics of a plant auxin-influx transporter. This biochemical characterization provides new insights and a novel tool for studying auxin entry into cells and its pivotal roles in plant growth and development.